The White House released a photo of a different bill signing from the same day relating to environmental regulations

From the link

What's worth noting, however, is that the White House - with its reluctance to provide evidence of the bill-signing - is acknowledging this could be a damaging for them. Presidential strategists probably envision photographs of the event in countless Democratic campaign adverts.

The president reportedly told aides after Parkland that they have to "do something" in response. The White House's reluctance to own up to its only legislative action on firearms is another indication that, when it comes to gun control, they may feel the ground shifting beneath their feet.